河北高寺台超镁铁岩体中锇矿物的锇同位素组成初步研究

Preliminary results of Os isotopic ratios of osmium minerals from the Gaositai ultramafic complex

报道了河北高寺台超基性岩体中的等轴锇铱矿和六方铱锇矿的Os同位素组成,两样品的187Os/188Os值分别为0.123 582±7和0.123 731±7.数据的解释有赖于年龄的和/或源区性质的制约.若已知矿物源区,可由同位素组成制约其形成时代.如上述矿物的锇来自于原始上地幔,其形成时代可能为500±10 Ma;若其源区为亏损的岩石圈地幔,其形成时代可能年轻到晚中生代;若源区为富集的地幔柱,其形成时代可能老到中元古代.反之,若能限定矿物形成时代,则可制约其源区性质.若上述锇矿物形成于印支期,则很可能源于亏损的岩石圈地幔.若它们形成于中元古代,则其源区很可能来源于富集的地幔柱性质的地幔.根据区域地质情况,我们倾向于这些Os来自富集地幔、高寺台岩体形成于中元古代的解释,但进一步解释有待于新的独立证据.尽管如此,这些数据对今后高寺台岩体的成因或年龄研究有参考价值.

Os isotopic compositions of an iso-osmiridium and a hex-osmiridium in the Gaositai ultramafic complex, Hebei province, were reported. The 187^Os/188^Os ratios for the two minerals are 0. 123 582±7 and 0. 123 731±7, respectively. The explanation of the data depends on other geological and geochemical constraints, such as the ages of the minerals and the characteristics of the source regions. Suppose the osmium was derived from a primitive upper mantle, the model ages of the two minerals could be 500±10 Ma; if the osmium was from an old depleted lithospheric mantle, the model ages could be as young as 60±15 Ma; while if the osmium was from fertile plume source, the model ages could be as old as 1 600± 10 Ma. The model age of the minerals depends on the degrees of depletion or of enrichment of the mantle source. In contrast, if the minerals were formed in Yindo-sinian orogeny, Os could be likely derived from an old depleted lithosphric mantle and if the minerals were formed in the Mid-Proterozoic, Os were probably derived from a fertile plume source. Based on regional geology, we prefer that the Os was derived from a fertile source and the Gaositai intrusion was formed during the Mid-Proterozoic. However, the issue merits further investigation. Nevertheless, the data provided by this work will help with understanding the age of Gaositai platinum-bearing intrusion and the mantle characteristics beneath the northern margin of the North China Block in the future.